The present application relates generally to operational amplifiers, and more specifically to output stages of MOS operational amplifiers.
Metal Oxide Semiconductor (MOS) operational amplifiers (op amps) are known that employ output stages of the class-AB type to achieve good output current capability. For example, a conventional class-AB MOS output stage may include a p-channel MOS (PMOS) output transistor and an n-channel MOS (NMOS) output transistor operating in a common source mode, a biased class-AB control circuit coupled between the gates of the PMOS and NMOS output transistors, and a current source coupled between the gate of the PMOS output transistor and a positive power supply. In this configuration, the gate of the NMOS output transistor is employed as an inverting input, and a common source connection of the output transistors is employed as an output of the output stage. In the event the conventional class-AB MOS output stage is driven by a differential stage providing differential outputs, the current source may be replaced by a current mirror, and the input of the current mirror may serve as the non-inverting input of the output stage.
Although the conventional class-AB MOS output stage described above generates sufficient output current for many applications, the MOS output stage has drawbacks. For example, the gain of an MOS op amp including the MOS output stage is typically limited by a number of factors such as (1) the output conductance of the input signal current source, (2) the output conductance of the current mirror, and (3) the impact ionization phenomenon, which generally causes leakage currents to be generated in MOS transistors operating under high drain-bulk voltages. Such impact ionization can cause unwanted parasitic conductance at the high impedance gates of the MOS output transistors.
Another drawback of the conventional class-AB MOS output stage is that it often generates a significant amount of distortion, i.e., its output is frequently non-linear. Like the gain limitations of MOS op amps incorporating the conventional MOS output stage, the non-linearity of the output stage also results from a number of factors. First, the output voltage of the MOS output stage is typically a non-linear function of the gate voltages of the respective PMOS and NMOS output transistors. Because of these non-linear gate voltages and the parasitic conductance of the high impedance gate nodes, a non-linear current is often required at the input of the output stage to offset the non-linear output. Further, there is typically significant non-linear parasitic capacitance at the high impedance gates of the output transistors. Such non-linear parasitic capacitance can be exacerbated by the parasitic capacitance of (1) the input signal current source, (2) the class-AB control circuit, and (3) the current mirror included in the output stage.
One way of canceling at least a portion of the parasitic conductance found in the conventional class-AB MOS output stage is to employ a current mirror with tracking feedback. However, although tracking feedback may be effective in canceling the output conductance of the current mirror, it has essentially no beneficial impact on the output conductance of the input signal current source and the non-linear parasitic conductance/capacitance of the output transistor gates.
Still another drawback of the conventional class-AB MOS output stage is that the circuitry for biasing the class-AB control circuit typically fails to provide for stable quiescent currents at the MOS output transistors, especially when short channel output devices are employed for high speed, high current applications.
It would therefore be desirable to have an output stage for an op amp that provides for higher gain and reduced distortion, while avoiding the drawbacks of the above-described conventional op amp output stages.